1. Field of the Invention
The present invention relates to fiber optic gyroscopes and more particularly to closed loop fiber optic gyroscopes.
2. Description of Related Art
The basic configuration of the interferometric fiber optic gyroscope (IFOG) was developed at Stanford University in 1981. The configuration of the basic Stanford interferometric fiber optic gyroscope is depicted in FIG. 1. A light source passes light through the interferometer optics and is split into two beams that propagate around the fiber optic coil of the gyro in opposite directions. The light is then recombined and the resultant intensity at the detector is proportional to the phase difference between the two beams. The phase difference between the two beams is in turn proportional to an input rotation rate about the input axis of the gyro. In the absence of rotation, the interferometer optics insure that both beams traverse the same optical path and thus yield a nominally zero bias for the gyro. In the basic Stanford configuration a phase modulator was constructed of a cylindrical piece of piezo-electric crystal with optical fiber wrapped around it which was placed to one side of the fiber optic sensing coil of the gyro. Applying a sinusoidal voltage to the crystal causes the crystal to expand and contract, thus lengthening and shortening the fiber, which in turn induces phase modulation onto the light traveling through the optical circuit of the gyro. The output of the gyro is then synchronously demodulated at the first harmonic. Synchronous demodulation at the first harmonic transforms the output of the gyroscope from that of a raised cosinusoid to a sinusoidal scale factor. The sinusoidal scale factor is more desirable because of the slope through zero, which is attractive from a controls standpoint, and is anti-symmetric, thereby making it possible to tell the direction that the gyro is rotating. The effect of the synchronous demodulation of the photo diode output is shown in FIG. 2. Thus, in the prior art, Stanford University defined a reciprocal optical circuit and demonstrated the need for waveguide-type optical devices to achieve inertial grade performance. In fact, Stanford University fabricated the entire interferometric fiber optic gyro from a single length of fiber optic waveguide. The Stanford interferometric fiber optic gyroscope is more fully described in the Stanford Report No. 3586 of June 1983 by Ralph Alan Bergh G. L., of Stanford, Calif., incorporated by reference herein.
The Stanford device as well as others of the prior art did not, however, address the additional problems which are now addressed by the present invention. The scale factor of the gyroscope, although superior to the raised cosinusoid, is still sinusoidal. A linear scale factor is highly desirable. Thus, the Stanford gyro did not provide the most desirable end result. Further, the Stanford gyro output is a simple analog dc voltage whereas a digital output is preferred. The fiber optic components used in the prior art interferometric fiber optic gyro are mechanically unstable and impractical in a hostile environment, as configured in the prior art devices.